1. Field of the Invention
The present invention relates to an active material and a method of manufacturing the active material.
2. Related Background Art
A polyanionic compound represented by LiNaMXO4 (where a is 1 or 2, M denotes one species selected from the group consisting of Fe, Mn, Co, Ni, and VO, and X denotes one species selected from the group consisting of P, Si, S, V, As, and Ti) is a promising positive electrode active material which can reversely insert and de-insert lithium. This polyanion compound has been known to exhibit a plurality of crystal structures such as triclinic (α-type) and orthorhombic (β-type) crystals and have different electrochemical characteristics according to the crystal structures.
In the polyanionic compound, electrons are strongly attracted to a tetrahedral skeleton of its crystal lattice, whereby metal atoms are isolated in the crystal lattice. Hence, the polyanionic compound has an ion binding property. Therefore, the polyanionic compound exhibits an electronic conductivity lower than that of other typical positive electrode active materials. Lithium-ion secondary batteries using such a polyanionic compound having a low electronic conductivity as a positive electrode active material have been problematic in that they fall short of yielding a capacity fully on a par with a theoretical capacity or exhibit a rate characteristic lower than that in the case using an Li-based laminar compound as a positive electrode active material.
An example of methods for imparting electronic conductivity to the polyanionic compound is one which combines the polyanionic compound with an electro-conductive agent such as carbon. Specific examples include a method of mixing a particle made of the polyanionic compound with carbon, a method of mixing a polyanionic compound precursor with carbon at the time of making the polyanionic compound, and a method of firing the polyanionic compound together with carbon, so that the polyanionic compound carries carbon (see Japanese Patent Application Laid-Open No. 05-159807 (Literature 1), Japanese Patent Application Laid-Open No. 11-329427 (Literature 2), Japanese Patent Application Laid-Open No. 2002-110163 (Literature 3), Japanese Patent Application Laid-Open No. 2003-203628 (Literature 4), Japanese Patent Application Laid-Open No. 2003-292308 (Literature 5), Japanese Patent Application Laid-Open No. 2003-292309 (Literature 6), Japanese Patent Application Laid-Open No, 2004-063386 (Literature 7), and Japanese Patent Application Laid-Open No. 2007-087841 (Literature 8)).
As a method for improving the capacity and rate characteristic of a lithium-ion secondary battery, Japanese Patent Application Laid-Open No. 2004-303527 (Literature 9) discloses a method using an electrode active material for a nonaqueous electrolytic secondary battery, which is a lithium-vanadium-phosphorus mixed compound powder having an orthorhombic LiVOPO4 crystal structure.
As a method for improving the cycle characteristic of a lithium-ion secondary battery, Japanese Patent Application Laid-Open No. 2001-68304 (Literature 10) discloses a method using an electrode active material for a nonaqueous electrolytic secondary battery, which is constituted by a vanadium-phosphorus mixed compound having a ω-VOPO4 crystal structure.
As a method for improving the initial capacity and discharge rate characteristic of a positive electrode of a lithium secondary battery, Japanese Patent Application Laid-Open No. 2003-173777 (Literature 11) discloses a method of combining a positive electrode active material for a nonaqueous lithium secondary battery with a conductive auxiliary characterized by firing a mixture of a transition metal compound and a lithium compound and combining thus obtained positive electrode active material with the conductive auxiliary by using a spray dryer.